The overall goal of the principal investigator is to relate the depression of ventricular function in human and experimental canine dilated cardiomyopathy to alterations in the mechanical behavior of the cardiac myofibril. Previous experiments by the applicant have demonstrated that the calcium sensitivity of isometric tension is increased in both forms of heart failure, and suggested that this increase is due to reductions in b- adrenergically mediated (protein kinase A-dependent) phosphorylation of myofibrillar proteins. Prior studies in normal myocardium suggest that protein kinase A-mediated phosphorylation of myofilament proteins results in important alterations in dynamic as well as isometric myofibrillar mechanical function, increasing unloaded or maximal velocity of shortening and accelerating the rates of tension development and myofibrillar relaxation. The central hypothesis of this proposal, therefore, is that reductions in the basal level of b-adrenergically mediated phosphorylation of myofilament proteins results in significant alterations in dynamic myofibrillar function, including reduced shortening velocities under physiologic loads, reduced maximal myofibrillar power output, and decreased rates of isometric tension development and myofibrillar relaxation. These hypotheses will be tested in myocyte-sized myofibrillar preparations from failing and non-failing human and canine myocardium, using techniques such as laser diffraction methods to monitor and control sarcomere lengths, and UV flash photolysis to rapidly change activator calcium concentrations. In the canine model of chronic rapid pacing-induced heart failure, changes in dynamic myofibrillar function will be directly related to alterations in myofibrillar protein phosphorylation, and to depression of left ventricular systolic and diastolic function. A second hypothesis is that chronic oral b-adrenergic blockade in dilated cardiomyopathy will limit the severity of contractile failure, increase basal phosphorylation of myofilament proteins, and normalize maximal myofibrillar power output and other indices of myofibrillar mechanical function in this model. This hypothesis will be tested in the canine chronic rapid pacing model using similar techniques.